Referring to FIG. 1, a conventional servo control system of an optical disc drive is shown. An optical disc 110 having a center hole is placed on a turn table 122. The turn table 122 is driven to rotate by a spindle motor 120 such that the optical disc 110 is rotated with the turn table 122. An actuator 180 outputs a driving force for an optical pickup head (PUH) 100 of the optical disc drive in a tracking (radial) direction or a focusing direction.
When an electronic signal is generated in response to the optical signal reflected from the optical disc 110 and received by the optical pickup head 100, a pre-amplifier 150 process the electronic signal into a series of servo error signals. The servo error signals include for example a radio-frequency signal RF, a sub-beam added signal (SBAD), a wobble signal, a focusing error signal FE, a tracking error signal TE and the like.
According to these servo error signals, the controller 170 generates a series of servo control signals to the actuator 180. Generally, the servo control signals include a tracking control signal and a focusing control signal. In addition, the actuator 180 includes at least a tracking coil and a focusing coil. According to the tracking control signal, the tracking coil generates a radial driving force for actuating a trace shift of the optical pickup head 100 in the tracking direction. According to the focusing control signal, the focusing coil generates a focusing-direction driving force for actuating a shift of the optical pickup head 100 in the focusing direction. Under the control of the radial driving force and the focusing-direction driving force, the optical pickup head 100 is located at the proper focusing position and onto the desired track.
As known, due to production failure or scratch, some defects are readily formed on the surface of the optical disc 110. Since data within the defective area of the optical disc 110 can not be read by the optical pickup head 100, the servo error signals outputted from the pre-amplifier 150 would become beyond expectation. The fluctuation of the servo error signals would further result in the unexpected servo control signals by the controller 170 and thus the actuator 180 may generate improper driving forces. Under this circumstance, the servo control system of the optical disc drive is unstable, which may cause the optical pickup head 100 to have focusing fail or slip track. Eventually, these defects could result in reading or writing errors.
FIG. 2 is a timing waveform diagram illustrating a relationship between a sub-beam added signal, a low-pass signal a defect signal. In the drawings, the term SBAD indicates a sub-beam added signal. The sub-beam added signal SBAD is filtered by a low-pass filter (not shown) to generate a low-pass signal (SBADLow-pass). Please refer to FIGS. 1 and 2. In a case that the optical pickup head 100 is moved along a track having a bright defect area with strong reflection, the sub-beam added signal SBAD rises up at a faster rate but the low-pass signal SBADLow-pass rises up at a slower rate. If the difference between the sub-beam added signal SBAD and the low-pass signal SBADLow-pass is larger than a first threshold value (M1), the defect signal (DETECT) outputted from the controller 170 is changed from a first level (e.g. a low level) to a second level (e.g. a high level). Until the difference between the sub-beam added signal SBAD and the low-pass signal SBADLowpass is smaller than the first threshold value (M1), the defect signal returns to the first level. On the contrary, in a case that the optical pickup head 100 is moved along a track having a typical defect area (e.g. a scratch), the sub-beam added signal SBAD drops down at a faster rate but the low-pass signal SBADLow-pass drops down at a slower rate. If the difference between the sub-beam added signal SBAD and the low-pass signal SBADLow-pass is smaller than a second threshold value (−M2), the defect signal (DETECT) outputted from the controller 170 is changed from the first level to the second level. Until the difference between the sub-beam added signal SBAD and the low-pass signal SBADLow-pass is larger than the second threshold value (−M2), the defect signal returns to the first level. According to the defect signal (DETECT), the driving force generated from the actuator 180 is adjusted in order to protect the optical disc drive from being subject to focusing fail or slip track.
FIG. 3 schematically illustrates a defect-detecting circuit disclosed in description of U.S. Pat. No. 6,882,611. As shown in FIG. 3, the defect-detecting circuit includes a low-pass filter 24, a subtractor 28, two comparators 22a and 22b, and an OR gate 26. A negative input end of the comparator 22a is coupled to a preset positive threshold (DFTH_P), which represents the first threshold value for detecting the bright defect. A positive end of the comparator 22b is coupled to a preset negative threshold (DFTH_N), which represents the second threshold value for detecting the typical defect.
The sub-beam added signal SBAD is filtered by the low-pass filter 24 to generate a low-pass signal (SBAD_LPF). The subtractor 28 subtracts the SBAD_LPF from the original SBAD and transmits the result to the two comparators 22a and 22b. If the result is larger than the first threshold value, the area is determined as a bright defect area. Whereas, if the result is smaller than the second threshold value, the area is determined as a typical defect area.
FIG. 4 is a timing waveform diagram illustrating related signals processed in the defect-detecting circuit of FIG. 3, in which a relatively long defect is present. In a case that the optical pickup head is moved along a track having a relatively long defect area (one kind of typical defect area), the sub-beam added signal SBAD drops down at a faster rate but the low-pass signal SBAD_LPF drops down at a slower rate. At the time spot T1, since the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is smaller than the second threshold value (DFTH_N), the defect signal (DETECT) is changed from the low level to the high level. Meanwhile, the detected area is determined as a typical defect area.
However, the above defect-detecting circuit usually generates inaccurate defect signals during the optical pickup head moves along the track having the long defect area. As the low-pass signal SBAD_LPF continuously drops down, the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF becomes larger than the second threshold value (DFTH_N) at the time spot T2. Meanwhile, the defect signal (DETECT) is changed from the high level to the low level and thus the defect signal (DETECT) is erroneously ended at the time spot T2. Under this circumstance, the defect-detecting circuit discriminates that no defect area is present.
At the time spot T3, since the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is larger than the first threshold value (DFTH_P), the defect signal (DETECT) is changed from the low level to the high level. Meanwhile, the detected area is determined as a typical defect area. Until the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is smaller than the first threshold value (DFTH_P) at the time spot T4, the defect signal (DETECT) is changed from the high level to the low level. In other words, an erroneous defect signal is generated from the time point T3 to T4.
As previously described, an erroneous defect signal is generated if a long defect is present. In a case that a long typical defect area is present (as shown in FIG. 4), two pulses sequentially including a typical defect area and a bright defect area occur. Moreover, in a case that a long bright defect is present, two pulses sequentially including a bright defect area and a typical defect area occur.
For avoiding generation of the erroneous defect signal, U.S. Pat. No. 6,882,611 disclosed a defect-detecting circuit and a defect-detecting method, and the contents thereof are hereby incorporated by reference. As shown in FIG. 5, the defect-detecting circuit includes a switch 51, a capacitor 30, a low-pass filter 24, a subtractor 28, an OR gate 26 and two comparators 22a and 22b. A negative input end of the comparator 22a is coupled to a preset positive threshold (DFTH_P), which indicates the first threshold value for detecting the bright defect. A positive end of the comparator 22b is coupled to a preset negative threshold (DFTH_N), which indicates the second threshold value for detecting the typical defect. The sub-beam added signal SBAD is filtered by the low-pass filter 24 to generate a low-pass signal (SBAD_LPF). The subtractor 28 subtracts the SBAD_LPF from the original SBAD and couples to the two comparators 22a and 22b. The switch S1 controls whether the SBAD is sent to the low-pass filter 24 and is controlled by the defect signal.
FIG. 6 is a timing waveform diagram illustrating related signals processed in the defect-detecting circuit of FIG. 5, in which a relatively long defect is present. In a case that the optical pickup head is moved along a track having a relatively long defect (one kind of typical defect), the sub-beam added signal SBAD drops down at a faster rate but the low-pass signal SBAD_LPF drops down at a slower rate. At the time spot T5, since the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is smaller than the second threshold value (DFTH_N), the defect signal (DETECT) is changed from the low level to the high level and the detected area is determined as a typical defect area. Once the defect signal (DETECT) is changed from the low level to the high level, the switch S1 is opened such that the SBAD signal can no longer flow through the low-pass filter 24 and the value of the low-pass signal (SBAD_LPF) is held by the capacitor 30.
Until the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is greater than the second threshold value (DFTH_N) at the time spot T6, the defect signal (DETECT) is changed from the high level to the low level. Meanwhile, the switch S1 is closed and the SBAD signal can flow through the low-pass filter 24.
In accordance with the defect-detecting circuit and the defect-detecting method shown in FIGS. 5 and 6, the area corresponding to the defect signal (DETECT) from the time spot T5 to T6 is determined as a typical defect area. Therefore, the long typical defect can be detected by using the second threshold value (DFTH_N) as a reference level. Similarly, the defect-detecting circuit can be used to detect the long bright defect by using the first threshold value (DFTH_P) as a reference level.
The defect-detecting circuit shown in FIG. 5 is usually implemented by an analog circuit. Since the analog circuit occupies much layout space of the integrated chip and fails to withstand process deviation, the application of this defect-detecting circuit is restrained. Moreover, it is difficult to precisely control the timing of the defect signal changing from the first level to the second level. That is, it is not flexible to adjust the time spots T5 and T6.
For solving the above-mentioned drawbacks, digitalized defect-detecting circuit and method are disclosed in description of US publication No. 20040145987, and the contents thereof are hereby incorporated by reference. As shown in FIG. 7, the defect-detecting circuit disclosed in US publication No. 20040145987 includes an A/D converter 200, a maximum value detector 202, a minimum value detector 204, a first comparator 206, a second comparator 208 and an OR gate 209. A first threshold value TH1 and a second threshold value TH2 are received by the first comparator 206 and the second comparator 208, respectively.
In the digitalized defect-detecting circuit, a radio frequency (RF) signal is converted by the A/D converter 200 into a digital format. The maximum value detector 202 detects an upper value of the digital RF signal, while the minimum value detector 204 detects a bottom value of the digital RF signal. The first comparator 206 compares the upper value of the digital RF with the first threshold value TH1. A logic value “0” is outputted if the upper value of the digital RF signal is larger than the first threshold value TH1, while a logic value “1” is outputted if the upper value of the digital RF signal is smaller than the first threshold TH1. The second comparator 208 compares the bottom value of the digital RF with the second threshold value TH2. A logic value “0” is outputted if the bottom value of the digital RF signal is smaller than the second threshold TH2, while a logic value “1” is outputted if the bottom value of the digital RF signal is larger than the second threshold value TH2. The output ends of the first comparator 206 and the second comparator 208 are coupled to the OR gate 209. According to the comparing results at the first comparator 206 and the second comparator 208, the OR gate 209 outputs a defect signal.
In the digitalized defect-detecting circuit of FIG. 7, asymmetry amount of the RF signal is calculated and used as an index for determining asymmetry degree of the RF signal. Since either the first threshold value TH1 or the second threshold value TH2 is used to determine one kind of defect (the typical defect or the bright defect), it is also difficult to precisely control the timing of the defect signal changing from the first level to the second level.
Therefore, there is a need of providing improved defect-detecting circuit and method so as to obviate the drawbacks encountered from the prior art.